In portable radio apparatuses, especially in mobile stations, the antenna requirements have become more severe. As the devices continue to shrink in size, the antenna naturally has to be small; preferably it is placed inside the covers of the apparatus. On the other hand, together with the introduction of new frequencies there has been a growing demand for mobile stations in which the antenna must function in two or more frequency bands. In addition, in dual-band antennas the upper operating band at least should be relatively wide, especially if the device in question is to be used in more than one system utilizing the 1.7 to 2-GHz range.
Antenna requirements may be met through various structural solutions. The solution according to the present invention is based on the application of a parasitic element in planar antennas. Several such structures are known in the art. Typically they comprise a printed circuit board with a ground plane on one surface and a conductive region connected to an antenna feed line and at least one parasitic conductive region on the other surface. Such a structure is shown in FIGS. 1a,b. FIG. 1a shows a top view of an antenna 100, and FIG. 1b shows a side view of a cross section of the same antenna. The structure comprises a dielectric plate 108. On the upper surface of the plate 108 there are conductive regions 120 and 130 which function as radiating elements. On the lower surface of the plate 108 there is a conductive region 110 which covers the whole surface and functions as a ground plane. The first radiating element 120 is connected at a point F through a feed conductor 102 to a source feeding the antenna. In addition, the element 120 is short-circuited to ground at a point S through conductor 103 so as to improve the electrical characteristics, such as impedance matching, of the antenna. The resulting structure is called a planar inverted F antenna (PIFA). The second radiating element 130 is parasitic, i.e. there is only an electromagnetic coupling between it and the first element 120. It, too, may have a short-circuit point. The purpose of the parasitic element is to further improve the electrical characteristics, such as bandwidth or radiation pattern, of the antenna.
One drawback of the above-described antennas according to the prior art is that their bandwidth is not always large enough for modern communications devices.
Radiating elements may be designed such that the bandwidth is increased through two adjacent resonance frequencies, but then the disadvantage of the structure is that the structure is relatively complex as regards ensuring reliable operation. An additional disadvantage of an element, which has two adjacent resonances, is that the polarization of its radiation rotates inside the band. Moreover, it is a disadvantage of the structures described above that they are sensitive to the effect of the user's hand, for example. If a finger, for instance, is placed over the radiating element of a PIFA on the outer cover of the apparatus, the operation of the PIFA will be impaired.